Controlled Oxidation and Self-Passivation of Bimetallic Magnetic FeCr and FeMn Aerosol Nanoparticles

Nanoparticle generation by aerosol methods, particularly spark ablation, has high potential for creating new material combinations with tailored magnetic properties. By combining elements into complex alloyed nanoparticles and controlling their size and structure, different magnetic properties can be obtained. In combination with controlled deposition, to ensure nanoparticle separation, it is possible to minimize interparticle interactions and measure the intrinsic magnetic property of the nanoparticles. Most magnetic materials are highly sensitive to oxygen, and it is therefore crucial to both understand and control the oxidation of magnetic nanoparticles. In this study, we have successfully generated oxidized, bimetallic FeCr and FeMn nanoparticles by spark ablation in combination with a compaction step and thoroughly characterized individual particles with aerosol instruments, transmission electron microscopy and synchrotron-based X-ray photoelectron spectroscopy. The generated nanoparticles had an almost identical transition-metal ratio to the electrodes used as seed materials. Further, we demonstrate how the carrier gas can be used to dictate the oxidation and how to alternate between self-passivated and entirely oxidized nanoparticles. We also discuss the complexity of compacting alloyed nanoparticles consisting of elements with different vapor pressures and how this will affect the composition. This knowledge will further the understanding of design and generation of complex alloyed nanoparticles based on transition metals using aerosol methods, especially for the size regime where a compaction step is needed. As a proof of concept, measurements using a magnetometer equipped with a superconducting quantum interference device were performed on samples with different particle coverages. These measurements show that the magnetic properties could be explored for both high and low surface coverages, which open a way for studies where interparticle interactions can be systematically controlled

Effect of the carrier gas on the structure and composition of Co–Ni bimetallic nanoparticles generated by spark ablation

Spark ablation is a versatile technique for producing pure size-selected nanoparticles. The carrier gas used in spark ablation affects the nanoparticles’ generation, crystalline structure, and chemical composition. The comprehension of this phenomenon can contribute to the design of nanoparticles with tailored properties. In this paper, we evaluate the effects of reducing (95%N2 + 5%H2), inert (N2), and oxidative (air) carrier gases in a spark ablation setup with Co–Ni alloyed electrodes. The agglomerates’ particle size distribution, morphology, structure, and composition were highly dependent on the carrier gas, especially its relative oxygen content. The agglomerates were then sintered into compacted particles. Three different crystalline structures and chemical compositions were observed with X-ray diffraction and confirmed with transmission electron microscopy for the compacted particles. For 95%N2 + 5%H2 and air, single-phase (Co,Ni) and (Co,Ni)O particles were identified, respectively, whereas for N2, two-phase (Co,Ni) and (Co,Ni)O particles were obtained. This work opens up new possibilities of tuning the structure and composition, i.e., distribution of metallic and oxide phases, of the produced particles and thus tailor their properties for specific applications by simply changing the carrier gas

In situ observation of synthesized nanoparticles in ultra-dilute aerosols via X-ray scattering (vol 12, pg 25, 2019)

The article In situ observation of synthesized nanoparticles in ultradilute aerosols via X-ray scattering, written by Sarah R. McKibbin, Sofie Yngman, Olivier Balmes, Bengt O. Meuller, Simon Tagerud, Maria E. Messing, Giuseppe Portale, Michael Sztucki, Knut Deppert, Lars Samuelson, Martin H. Magnusson, Edvin Lundgren, and Anders Mikkelsen, was erroneously originally published electronically on the publisher's internet portal (currently SpringerLink) on 3 September 2018 without open access. The copyright of the article changed in November 2018 to (c) The Author(s) 2018 and the article is forthwith distributed under the terms of the Creative Commons Attribution 4.0 International License (https://doi.org/creativecommons.org/licenses/by/4.0/), which permits use, duplication, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.The original article has been corrected

In situ observation of synthesized nanoparticles in ultra-dilute aerosols via X-ray scattering

In-air epitaxy of nanostructures (Aerotaxy) has recently emerged as a viable route for fast, large-scale production. In this study, we use small-angle X-ray scattering to perform direct in-flight characterizations of the first step of this process, i.e., the engineered formation of Au and Pt aerosol nanoparticles by spark generation in a flow of N2 gas. This represents a particular challenge for characterization because the particle density can be extremely low in controlled production. The particles produced are examined during production at operational pressures close to atmospheric conditions and exhibit a lognormal size distribution ranging from 5–100 nm. The Au and Pt particle production and detection are compared. We observe and characterize the nanoparticles at different stages of synthesis and extract the corresponding dominant physical properties, including the average particle diameter and sphericity, as influenced by particle sintering and the presence of aggregates. We observe highly sorted and sintered spherical Au nanoparticles at ultra-dilute concentrations (< 5 × 105 particles/cm3) corresponding to a volume fraction below 3 × 10–10, which is orders of magnitude below that of previously measured aerosols. We independently confirm an average particle radius of 25 nm via Guinier and Kratky plot analysis. Our study indicates that with high-intensity synchrotron beams and careful consideration of background removal, size and shape information can be obtained for extremely low particle concentrations with industrially relevant narrow size distributions. [Figure not available: see fulltext.]